The invention relates to low noise miniature transformers that are useful in hybrid integrated circuits, and more particularly to planar transformers having multiple, series-connected primary windings and a non-overlapping secondary winding imprinted on various layers of a circuit board so as to minimize electromagnetic radiation and electric field coupling between primary and secondary windings, with a ferrite core formed by two ferrite sections, each including a thin, flat plate separated from the other by two spacer sections and also supported by the circuit board.
There are a variety of electronic products which are powered by rechargeable batteries. The functional operation of some electronic products, including some powered by rechargeable batteries, is made unreliable by the presence of electrical noise. FCC regulations limit the amount of electromagnetic interference that can be radiated or conducted to the AC main power line by electronic devices. In some products, the presence of electrical noise causes sounds which are distracting to the user. Portable cellular telephones are an example of a noise-sensitive product. Some such products contain integral battery chargers that convert AC line voltage to low DC voltage levels at which conventional integrated circuit components can operate. It is important that the integral battery charger not introduce sufficient electrical noise to interfere with the reliable operation of the portable cellular telephone, not interfere with other products sharing the charger line voltage, or not violate FCC standards which are applied to electrical products to protect consumers from potential problems. There is an unmet need for a smaller, high efficiency, high frequency, low noise transformer that can be incorporated as part of an AC-to-DC converter or other circuit in a small hybrid integrated circuit package, such as a DIP package, or in a power connecter assembly. Hybrid electronic integrated circuits often are constructed using transfer molded packaging techniques to produce low cost electrical/electronic functions in a single conventional package, such as a dual-in-line package (DIP). Inclusion of magnetic components such as transformers in hybrid integrated circuits has always presented a major challenge, because the transformer cores required usually have large cross-sectional areas. Such large cross-sectional areas are inconsistent with the need to provide circuit functions in small packages.